I'm not sure I would tend to think that it's only when the next gen systems will launch that manufacturers should safely expect all theirs costumers to own an HDTV and I don't speak about fullHD.
People don't change their tv that often especially when they just bought a brand new HD 60" lcd (even if it's not full hd).
By the way manufacturers could be on a tigher transistor budget for the next gen system, i would be really sad if these systems to begin with have to deal with twice the pixel count.
Of course everyone won't own HD systems around 2011, a fairly large number will still be using SD. Look, PAL and NTSC has been around for a loooong time, with colour support tacked on along the way. 1920x1080p is the successor, and it will likewise be with us for a long time. The in-between formats are there for marketing reasons (selling the same product one more time) and for technical reasons, mainly concerning broadcasting bandwidth.
The 1366*768 resolution devices aren't less expensive to produce, the format has simply been there to produce tiers in the marketplace. From a technical standpoint, it is completely artificial, and it is showing signs of dissolving. 1920x1080 devices are close to price parity and the market are finding new avenues of producing pricing strata - sizes, 100/120 Hz, ambient light sensing, crippling input options, upcoming technologies such as LED backlighting, OLED screens... Never fear, the consumer will still be milked, just using something other than resolution.
I have no personal stake in this, just relatively old age - 1366x768 has pretty much served its purpose. Once price parity is a reality (as it should be since production costs are basically identical) noone will have any incentive to either buy or sell this interim format. It's a blip in history. Those who have it will continue to enjoy their displays until they find reason to exchange them, but it will be gone from the market.
3-4 years or so down the line, and from there on out, I can't see why you'd target anything other than 1920x1080. Everyone can still enjoy the content in down converted form if need be. If developers run out of pixel pushing power, rendering in 960x540 and converting from there would be an option. But I hope that won't be necessary, from a technical perspective resolution is a very simple problem requiring only brute force, and by 2011-2012, this should be doable at low cost and power.
Hell, I'm willing to bet even Nintendo will do it.